Plural purpose sludge incinerating and treating apparatus and method

ABSTRACT

Apparatus and procedure utilizing only a single furnace for incinerating and recalcining sewage sludge and lime sludge obtained from a tertiary raw sewage treating procedure. The carbon dioxide level adjacent the discharge end of the furnace is minimized to obviate recarbonation of the recalcined lime particles discharged from the furnace. The lime particles and ash particles are allowed to form in the furnace in their natural sizes so that they may be separated mechanically. Alternatively, the ash particles and lime particles are slaked together for reuse in the sewage treatment procedure.

United States Patent [72] lnventors Paul]. Cardinal, Jr.; 2,072,1543/1937 Butterfield 210/45 kobel'll-sherwood,bothofBrisbane, 3,279,60310/l966 Busse 210/67 Calif. 3,345,288 10/1967 Sontheimer. 210/10 .[21]Appl. No. 807,254 3.440.165 4/1969 Davis etal. 210/45 22 Fi d M 14,19 93,440,166 4/1969 Davis et al. 210/45 X [45] Patented Nov. 30, 1971 OTHERREFERENCES [73] Ass'gnee Ellylmtechsyftemstlnc' Babbitt. H. E., Sewerageand Sewage Treatment, Sixth Brlsbane, Edit, 1947. John Wiley & Sons, N.Y., pp. 536 and 537. Re-

lied on (Copy in GP. 176) [54] PLURAL PURPOSE SLUDGE lNClNERATlNG AND Py Examine -Michael Rogers TREATING APPARATUS AND METHOD Attorney-Flehr.Hohbach, Test, Albritton & Herbert 15 Claims, 2 Drawing Figs. [52] US.Cl 210/6, ABSTRACT: Apparatus and procedure utilizing only a Single IH0/82m/67'210/152 furnace for incinerating and recalcining sewage sludgeand [51] lnt.Cl C02c5/02 lime s|udge b i d from a tertiary raw sewagetreating [50] Fleld ofsflllch 210/6, 10, procedure The carbon dioxidelevel adjacent the discharge 252; 110/8 13 end of the furnace isminimized to'obviate recarbonation of the recalcined lime particlesdischarged from the furnace. The [56] References cued lime particles andash particles are allowed to form in the fur- UNITED STATES PATENTS nacein their natural sizes so that they may be separated 2.015.050 9/1935Baird et al. 1 10/13 X mechanically. Alternatively. the ash particlesand lime particles are slaked together for reuse in the sewage treatmentprocedure.

LIME 2-1 3 EMAGE PRIMARY SECONDARY i CLARIFIER I TREATMENT 9 9 RETURNACTIVATED SLUDGE 'i EXHAUST |2 CENTRATE GASES l I I g) I 23 THICKENERDEWATERER I4 OVERFLOW I 13 J\ l l INCINERATOR 27 8: COOLER CALCINER 2sa:

Z ALTERNATE 26 WASTE souos 36 33 STORAGE 29 CHEMICAL l RE-USE 32 SLAKERl CulOl-ll 3 7 1y coo CALCINED LIME 8 I ASH PARTICLES ASH C00 FSEPARATOR '--ASH WASTE PATENTED HUV30IHYI SEWAGE SHEEI10F2 LIMESECONDARY SECONDARY PRIMARY INFLUENT' I 7 L EFFLUENT CLARIFIER TREATMENTCLARIFIER H RETURN ACTIVATED SLUDGE EXHAUST |6 ..T CENTRATE K GASESTHICKENER DEWATERER r OVERFLOW I3 INCINERATOR 8: CALCINER ALTERNATEWASTE souos STORAGE CHEMICAL RE-USE SLAKER CALClNED LIME 8: ASHPARTICLES ASH CO0 SEPARATOR T as v ASH WASTE INVIJNTORS PAUL J. CARDINALJR.

ROBERT J. SHERWOOD 1M, T T W TTORNEYS PATENTEDIIIIvwIIIII 3523 75 SHEET2 III 2 CONCENTRATED 00 LIME SLUDGE CAKE I "*1 I-2I LIME a SEWAGE ISCRUBBER SLUDGE CAKE 1 WEHEEQKE l7 v 22 EXHAUST GASES INCINERATORCALCINER FUELT-fi 7 CALCINED LIME 2s &

ASH PARTICLES I TO SLAKER OR SEPARATOR INVIZNTORB PAUL II. CARDINAL JR.BY ROBERT J. SHERWOOD PLURAL PURPOSE SLUDGE INCINERATING AND TREATINGAPPARATUS AND METHOD BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to the field of raw sewage treatment.More particularly, this invention relates to the heat treating andrecalcination of lime sludge and sewage sludge produced duringphosphorous removal by lime precipitation in a tertiary sewage treatmentplant. In its more particular aspects, this invention relates to thefield of treatment of ash and calcined lime obtained from a recalciningincinerator, and to the separation of the ash and calcined lime in amechanical separator for selective reintroduction of the lime into thesewage treating procedure. In an alternate embodiment, this inventionrelates to the field of slaking both ash and recalcined lime particlestogether for selective reintroduction into and chemical reuse in the rawsewage treating procedure.

2. Description of the Prior Art Tertiary sewage treating procedures arewell known in general terms, as is the recalcining of sewage and linesludge resulting from such a tertiary treating procedure. However, sofar as is known, the particular recalcining and treating procedure ofthis invention, in which the carbon dioxide level adjacent the dischargeend of a recalcining furnace is minimized to obviate the possibility ofrecarbonation of recalcined lime, has not been utilized heretofore.Furthermore, so far as is known, mechanical separation of ash andcalcined lime particles which have been permitted to develop to theirnormal size within the heating device similarly has not been utilizedheretofore in the manner described. Additionally, so far as is known,the alternative procedure disclosed herein of introducing both calcinedlime particles and ash particles directly into a slaker for treatmenttherein, and the subsequent introduction of the slaked product back intothe raw sewage treating system for chemical reuse therein, has not beenutilized heretofore.

While mechanical separation of ash and calcined lime has been disclosedgenerally heretofore, such as discussed in Albertson US. Pat. No.3,409,545, assigned to Dorr-Oliver Incorporated, such patent and otherknown commercial operations do not utilize the specific proceduralfeatures recited herein.

SUMMARY OF THE INVENTION This invention relates generally to an improvedprocedure and apparatus combination for treating line sludge and sewagesludge obtained from a conventional raw sewage treating procedure, suchas a tertiary treating procedure. More particularly, this inventionrelates to the heat treating and recalcining of lime sludge and sewagesludge obtained from a tertiary treating procedure in which gaseouscombustion products, such as carbon dioxide, are selectivity maintainedat a low level adjacent the discharge end of the heating apparatus toeliminate or obviate the possibility of recarbonation of the recalcinedlime produced in and discharged from the heating apparatus. Suchselective limitation of the carbon dioxide level is achieved by omittingheat input sources adjacent the discharge end of the heating apparatus.

Still more particularly, this invention relates to a procedure andapparatus for producing calcined lime particles and ash particles ofdiffering sizes and specific gravities so that such particles may beselectively separated from each other, based on their respective sizesor specific gravity, in a mechanical procedure following their dischargefrom a recalciner. In an alternative embodiment, this invention furtherrelates to a procedure and apparatus for slaking both calcined limeparticles and ash particles together and for introducing such combinedparticles after slaking back into a raw sewage treating procedure forchemical reuse therein.

Preferably, prior to slaking or separation, the calcined lime and ashparticles are cooled and, if desired dry stored until needed forsubsequent procedural steps in the treating operation.

According to the foregoing, objects of this invention include: theprovision of an improved method for heat treating and recalcining limesludge and sewage sludge obtained for a tertiary treating sewageprocedure; the provision of a process and apparatus for controllingrecarbonation of recalcined lime particles obtained in a heating device,such as a recalcining furnace; the provision of an incinerating andcalcining procedure in which calcined lime particles and ash particlesare permitted to develop to their natural sizes in a recalcining furnaceto facilitate their subsequent mechanical separation; the provision ofan apparatus and process for slaking both calcined lime and ashparticles simultaneously, and for reusing such slaked particleschemically in a raw sewage treating procedure; and the provision of aprocedure and recalcining furnace for selectively controlling carbondioxide adjacent the furnace discharge end.

These and other objects of this invention will become apparent from astudy of the following disclosure in which reference is directed to theaccompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. I is a schematic view of the limesludge and sewage sludge treating apparatus and procedure embodied inthe present invention, illustrated in conjunction with a raw sewagetreating procedure.

FIG. 2 is a schematic view of the incinerating and calcining apparatusand procedure forming part of the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Removing phosphate from rawsewage is a major consideration in sewage treating plants of thetertiary treatment type. Known methods of handling treated sewage wastesheretofore commonly included utilization of two separate heating units,such as furnaces, one for recalcining the lime sludge produced duringprecipitation of phosphates from the sewage, and the other forincinerating the other waste sewage solids produced during the sewagetreatment.

An important feature of this inventionlresides in the utilization ofonly a single-heating unit, suchas a multiple-hearth furnace, in whichlime sludge and sewage sludge are simultaneously heat treated torecalcine the lime sludge and to incinerate the sewage sludge waste tothe fullest extent possible. Advantages of such a simplified system areits substantial savings in cost due to the elimination of one furnacefrom the treating plant, thus reducing substantially the initial capitalcost requirements. Additional operating cost savings are realized due toreduced fuel consumption by operating a single furnace to attain thesame results previously produced in two separate furnaces.

Furthermore, a higher quality product is obtained in the subjectinvention by utilizing a modified incinerating and recalcining furnacein which input heating devices are eliminated adjacent the discharge endof the furnace from which recalcined lime particles and ash aredischarged. With conventional double fumace arrangements utilizedheretofore, the lime recalcining operation was accompanied by parallelflow between the combustion gases, notably carbon dioxide, and therecalcined lime particles being discharged from the furnace. The contactof high level carbon dioxide waste gases with the recalcined limeparticles in a conven tional procedure frequently resulted inrecarbonation of the recalcined lime particles and discharge of a lessthan satisfactory product from the furnace.

By utilizing a multiple-hearth furnace, and by positioning gas burnersor like heat input sources on all but the hearth adjacent the dischargeend of the furnace, the carbon dioxide content in the lower dischargezone of the furnace is maintained at a minimum, thereby eliminating orobviating the danger of recarbonation of the recalcined lime andensuring a higher quality discharge product.

After discharge from the furnace, the combined incinerated ash andrecalcined lime particles may be slaked together or,

alternatively, mechanically separated so that the calcium oxide may beseparately slaked. In either event, the slaked product preferably isreintroduced into the tertiary treating procedure where its chemicalvalue is reutilized.

The recalcined lime from the furnace may be reused either in the primarystage, in a mixer following the secondary treatment, or as a split feedto both the primary and secondary stages, as may be desirable for mosteffective results in a par ticular sewage treating plant.

The recalcining furnace can be supplied with a dewatered mixture of limesludge and sewage sludge from a single dewatering device or,alternatively, lime sludge and sewage sludge may be introducedseparately into the furnace from separate dewatering devices.

It has further been found that another important aspect of thisinvention resides in the combustion of lime sludge and sewage sludgecontaining calcium carbonate, organic solids,

and other volatile and inert solids, in the same furnace to producesterile particles of nonputrescible particles of ash and recalcinedlime. By utilizing an incinerating procedure employing a multiple-hearthfurnace in which recalcination and combustion is carried out, it ispossible to employ a subsequent mechanical separation for easilyseparating the recalcined lime particles from the ash particles.

Such separation may be effected by known mechanical devices, such as ascreening device or a cyclone centrifuge, which utilizes as itsseparating concept the different sizes or different specific gravity ofthe ash particles and recalcined lime particles produced in the furnace.It has been found that by allowing the ash particles and recalcined limeparticles to fonn naturally during incineration and recalcination, byminimizing the abrasion to which such particles are subjected duringtheir formation, the particles will form in differing size ranges ratherthan in a generally uniform or homogenous size range.

This invention recognizes and utilizes the operational characteristicsof a multiple-hearth furnace in which such normal particle size growthis encouraged to result. In many quarters, such characteristics of amultiple-hearth furnace have been considered disadvantageous rather thandesirable.

The low abrasive characteristics of a multiple-hearth furnace onparticles formed therein is in distinction to other treating procedures,such as cyclonic combustion chambers utilized in coal fired systems, inwhich the abrasive action of the particles being formed causes suchparticles to be produced in substantially uniform small physical size.For example, it has been found that in such other proceduressubstantially uniform particles within the to 50 micron size rangecommonly result.

In contrast, the incineration and recalcination procedure carried on ina multiple-hearth furnace does not result in formation of particles inhomogenous particle size ranges. The multiple-hearth particle sizes varyover substantial ranges, from low micron size particles to particles asmuch as several inches in diameter. Heretofore, such nonhomogenousparticle formation frequently has been considered an indication of poorcombustion but it has been found that such nonhomogeneity is a desirablefeature in incinerating sludges obtained from chemical treatment of rawsewage.

Because of the tendency of incinerated sludge in a multiplehearthfurnace to grow to its own natural constituent particle size range,physical separation of calcined particles from ash particles aftermultiple hearth burning is simplified and conventional mechanical orphysical means may be utilized. The compounds formed duringincineration, such as calcium oxide, and other compounds such as ironoxide, aluminum oxide and silicon oxide, all have natural size andspecific gravities characteristics peculiar to their own naturalphysical and chemical structure. When these materials are allowed tocombust in a system which does not physically abrade them to homogenoussizes with other particles of different constituency, it has been foundthat such particles achieve a close resemblance to their natural sizeranges. Thus, utilization of both the specific gravity and size ofparticles, such as calcium oxide particles, may be utilized tofacilitate mechanical separation thereof from other particles such aswaste ash.

Referring to FIG. 1 the improved treating process and apparatuscombination utilized in this invention are shown in conjunction with astandard raw sewage tertiary treating procedure in which raw sewageinfluent is introduced, through conduit 1, along with lime passingthrough conduit 2, into a primary clarifier 3. After clarification, theclarified mixture is subjected to a secondary treatment, such asaeration, in secondary treating tank 4, following which secondaryclarification in a secondary clarifier 6 normally is effected. Thetreated effluent passes from the system through conduit 7, forsubsequent disposal or valuable reuse, while activated sludge isselectively reintroduced from the secondary clarifier into the system inknown fashion through conduits 8 and 9 as illustrated.

The sewage discharge mixture from the primary clarifier 3, whichincludes lime sludge and sewage sludge in a fluid carrier, is introducedthrough conduit 11 into a thickener 12, the fluid overflow from whichpasses through conduit 13 for disposal. The thickened lime sludge andsewage sludge are then preferably introduced into a dewaterer 14, forfurther concentration therein. Such dewaterer may take any known fonn,such as a centrifuge, vacuum filter, or filter press, of readilyavailable type. In the described embodiment, a centrifuge dewaterer isutilized and the liquid centrate separated therein from the solids isdischarged through a conduit 16 for disposal.

As thus far described, the system constitutes a generally standard rawsewage tertiary treating procedure of the type known in the artheretofore.

The dewatered lime sludge and sewage sludge, in dewatered cake form, arethen introduced through conduit 17 into a heating device 18 in whichrecalcination of the lime sludge and incineration of the sewage sludgeare to be effected. The heat treating device preferably is amultiple-hearth furnace modified as described hereinafter to produce theimproved results characteristic of this invention.

FIG. 2 illustrates an alternate arrangement in which the lime sludgecake may be separately introduced into the furnace through a conduit 21while the sewage sludge cake may be separately introduced into thefurnace through conduit 22. ln such arrangement, the lime sludge andsewage sludge are separately dewatered in suitable devices. The sludgeinfeed arrangement chosen will be selected depending upon particularplant requirements.

Fuel is introduced into furnace 18 through a suitable fuel source,designated 19, for burning therein in known fashion. Preferably suchfuel is gas to feed the conventional gas burners spaced throughout thefurnace as described hereinafter.

The exhaust gases resulting from incineration of the sludge in furnace18 are discharged through conduit 23 from the top of the furnace andpreferably are passed through a scrubber 24 of any known construction tocleanse the carbon dioxide produced during combustion for subsequentreuse thereof. As noted from FIG. 1, such exhaust gases, particularlythe concentrated carbon dioxide, may be selectively reintroduced intothe tertiary treating system through an extension of conduit 23 forutilization in the treating procedure for the purposes describedhereinafter.

Preferably the multiple-hearth furnace is gas fired and includes burnersselectively positioned along its length adjacent all hearths of thefurnace except the hearth which is most closely adjacent the lowerdischarge end of the furnace. By eliminating the burner adjacent thefurnace discharge end, it has been found that continued combustion isnot encouraged at such discharge end so that the carbon dioxide level inthe lower discharge zone of the furnace is minimized to ensure a higherquality product being produced in the furnace. That is, prior to itsdischarge from the furnace, the calcined lime produced is less subjectto recarbonation because it is generally out of contact with carbondioxide and other gases I calcined lime is minimized and the possibilityof recarbonation noted above is obviated.

As noted previously, a further important advantage obtained fromutilization of a multiple-hearth furnace results from the ability ofsuch furnace to produce naturally sized particles of calcined limeand-ash which may more readily be separated physically. That is, theminimization of abrasive action on the particles produced, which ischaracteristic of cyclone combustion chambers and the like, permitsnatural particle size growth, thereby simplifying physical separationsubsequent to discharge from furnace 18.

The principal products of the furnace, namely reclacined lime and ashparticles, are discharged from the furnace through conduit 26, desirablyinto a cooler 27 in which the temperature thereof is reduced tofacilitate subsequent handling thereof. Preferably, a substantially dustfree cooling procedure isutilized and for that purpose cooler 27preferably is of the type referred to as a rotating hollow disc cooler(THERMAL DISC cooler of the type produced by BSP Corporation) whichtreats powdery materials within an enclosed system to eliminate dusthandling and safety problems while efiecting efficient cooling. Such acooler is illustrated and described in Stevens US. Pat. No. 3,391,733dated July 9, 1968. Such a rotating disc cooler utilizes water as a heattransfer medium and effectively decreases the temperature of therecalcined particles and ash particles from the high temperature atwhich they exit'from the furnace to less then 400 F. From the cooler,the particulate product passes through conduit 28 into a dry storage bin29 for subsequent treatment or, alternatively, through conduit 31 to analternate waste solids disposal area.

From storage bin 29, the recalcined lime and ash particles are withdrawnas required and pass through conduit 32 for subsequent introduction andtreatment in a slaker 33. Altematively, the recalcined lime and ashparticles may be introduced through conduit 35 for handling in aseparator 34. The firstmentioned alternative treating procedure, namelyof slaking both the ash and the calcium oxide in the calcined limetogether, is a novel feature of the subject treating procedure when suchslakcd product is reintroduced through conduit 36 back into the tertiarytreating procedure, as noted from FIG. 1, for chemical reuse therein. Inknown fashion, in the slaker the ash and lime particles are watertreated to convert calcium oxide to calcium hydroxide suitable for reusein the sewage treatment.

The other alternative treating procedure shown in FIG. 1 employsselectively introducing calcined lime and ash particles into separator34 for physical separation from each other. Following such separationthe separated calcium oxide is rein troduced into the system throughconduit 37 in advance of the slaker for conversion therein to calciumhydroxide and subsequent reuse in the tertiary treatment procedure. Theseparated ash particles in turn are discharged through conduit 38 forsuitable ash waste disposal.

The ash-calcined lime separator selected may be of several availabletypes which operate on mechanical principles of separating particlesbased on their varying sizes or specific gravities or the like. As notedpreviously, the ash and calcined lime particles are allowed to achievetheir normal growth in the multiple-hearth furnace so that physicalseparation thereof is simplified. In its preferred aspects, theseparator is of the cyclone centrifuge type or the mechanical screentype in which particle specific gravity and size of the respectiveproducts are utilized to efiect their separation from each other. Bothtypes of equipment, namely centrifugal separation equipment and physicalscreening separation equipment, are

available on the market from companies such as Buell Engineering Companyor Bauer Brothers Company.

By way of specific examples, the plural purpose sludge burning proceduredescribed is set out with reference to particular furnace sludge chargesin the following two tabular examples. The sludge constituencies inexamples 1 and 2 are set out for reference points A, B and C appearingin FIG. 1, with reference point A being in advance of furnace 18,reference point B being adjacent the discharge end of the furnace l8,and reference point C being downstream from slaker 33.

DUAL LIME AND SEWAGE SLUDGE INCINERATING AND RECALCINING-EXAMPLES 1 AND2 A1 Ag B B; C C,

Item (lbs):

Volatile solids 1,250 l, 250 0 0 0 0 Inert solidsl 510 510 510 510 510510 CEO 0: 1, 260 2, 500 0 0 0 0 CasOH (PO03 275 550 0 0 0 0 Ca(0H) 0 00 0 950 1,860 a 0 0 716 1, 430 0 0 C8s(PO4)z 0 0 250 500 250 500 Totals(lbs.) 3, 285 4, 810 1, 475 2, 440 l, 710 2, 870

Percent CaO 0 0 49 69 0 0 During operation of the treating system undera relationship such as illustrated in example 2, the additional limeadded to the primary clarifier of the tertiary treating procedureresults in a pH therein of greater than 10. Such additional limeproduces no change in the incineration and recalcining proceduredescribed. To counteract the high pH the exhaust gases from the furnaceare reintroduced into the tertiary treating procedure in advance of thesecondary treatment as noted in FIG. 1. Without such a CO,reintroduction step, the bacteria in the secondary treatment operationwould be inhibited by exposure to too high a pH and the reintroductionof CO into the system lowers the pH to a workable level.

By way of further specific example, it has been found that in amultiple-hearth furnace of conventional six-hearth construction,temperature ranges preferably are modified from those utilizedheretofore in sludge burning. The following table sets out temperatureranges in a conventionally utilized furnace and in the improved modifiedfurnace of this invention. It will be noted that higher incineration andrecalcining temperatures are employed in the intermediate zone of thefurnace and the discharge temperatures are accordingly higher, due tothe slower cooling from the more elevated incineration temperaturesutilized in the intermediate furnace zone. However, the higher dischargetemperatures are not accompanied by high level carbon dioxide for thereasons advanced previously.

Having thus made a full disclosure of this invention, reference isdirected to the appended claims for the scope of protection to beafforded thereto.

We claim:

l. A method of heat treating in a single furnace lime sludge and sewagesludge obtained from a raw sewage tertiary treating procedure or thelike, comprising A. introducing said lime sludge and sewage sludge intoa furnace and moving the same therethrough at a predetermined rate,

B. incinerating said sludge in a first zone of said furnace andrecalcining said sludge in another zone of said furnace by thennallydecomposing said sludge as the same moves therethrough,

C. while said sludge is moving through said furnace, controlling theabrasive contacts to which said sludge is subjected in said furnace sothat generally discrete calcined lime particles and ash particles arepermitted to form naturally to their normally different sizes tofacilitate subsequent mechanical separation thereof, and

D. controlling the formation of and flow of the gases of combustion suchas carbon dioxide in said furnace by eliminating input heat sourcesadjacent the discharge end of said furnace so that such gases aremaintained at a low level adjacent such discharge end of said furnacefrom which said ash and recalcined lime particles are discharged toobviate recarbonation of such recalcined lime particles emanating fromsaid furnace.

2. The method of claim 1 in which said first incinerating zone of saidfurnace is maintained at a temperature not exceeding approximately l,650F. and in which said other calcining zone of said furnace is maintainedat a temperature of approximately L850 F.

3. The method of claim 1 which further includes E. dewatering said limesludge and sewage sludge prior to introducing the same into saidfurnace.

4. The method of claim 1 which further includes E. slaking saidrecalcined lime and ash particles to prepare said recalcined limeparticles for subsequent recycling and reuse in said raw sewage treatingprocedure.

5. The method of claim 1 in which said recalcined lime and ash particlesare mechanically separated following discharge from said furnace byutilizing their different particle sizes to effect such separation.

6. The method of claim I which further includes E. treating said ash andrecalcined lime particles discharged from said furnace to physicallyseparate the same from each other so that said lime particles may beselectively reused in treating raw sewage.

7. The method of claim 6 in which said slaked lime particles arereintroduced directly into said raw sewage tertiary treating procedure.

8. The method of claim 4 in which said separation of said ash andrecalcined lime particles is efiected physically by mechanicallyremoving said ash particles from said lime particles based on and byutilizing the different sizes of such particles following discharge ofthe same from said furnace.

9. The method of claim 1 in which said incineration and recalcinationare effected in a multiple-hearth furnace.

10. The method of claim 9 in which the temperatures in said furnace areregulated to divide said furnace into three predetermined operativesludge treating zones in which the temperatures are maintained withinpreferred ranges as follows:

Entrance zone approximately 900-l l 00 F. lnterrnediate zoneapproximately l,500l ,900" F. Discharge zone approximately 800-] 1 00 F.

fonned in said furnace so that generally discrete calcmed lrme particlesand ash particles may form naturally within said furnace to their normaldifferent sizes, and

B. a mechanical separator into which said particles of ash andrecalcined lime are selectively introduced following incineration andcalcining thereof in which said particles are separated from each otheron the basis of their different sizes.

12. The apparatus of claim 11 which further includes C. a dewateringdevice for removing excess liquid from said sewage sludge and limesludge prior to introduction thereof into said furnace.

13. The apparatus of claim 11 which further includes C. a slaker intowhich said ash and recalcined lime particles are introducible forslaking and subsequent reuse of said lime particles in said sewagetreating procedure.

14. The apparatus of claim 11 in which said furnace is of the multiplehearth type having fuel burners along each of its hearths except thehearth adjacent said discharge end, whereby combustion and the carbondioxide level adjacent said discharge end are minimized to obviaterecarbonation of the recalcined lime discharged from said furnace.

15. The apparatus of claim 14 in which said furnace is operativelydivided into at least two distinct zones for incinerating andrecalcining said sludge in which the temperatures are maintained at alevel not exceeding approximately l,650 F. in a first incinerating zoneand maintained at approximately l,850 F. in the second calcining zone.

2. The method of claim 1 in which said first incinerating zone of saidfurnace is maintained at a temperature not exceeding approximately1,650* F and in which said other calcining zone of said furnace ismaintained at a temperature of approximately 1,850* F.
 3. The method ofclaim 1 which further includes E. dewatering said lime sludge and sewagesludge prior to introducing the same into said furnace.
 4. The method ofclaim 1 which further includes E. slaking said recalcined lime and ashparticles to prepare said recalcined lime particles for subsequentrecycling and reuse in said raw sewage treating procedure.
 5. The methodof claim 1 in which said recalcined lime and ash particles aremechanically separated following discharge from said furnace byutilizing their different particle sizes to effect such separation. 6.The method of claim 1 which further includes E. treating said ash andrecalcined lime particles discharged from said furnace to physicallyseparate the same from each other so that said lime particles may beselectively reused in treating raw sewage.
 7. The method of claim 6 inwhich said slaked lime particles are reintroduced directly into said rawsewage tertiary treating procedure.
 8. The method of claim 4 in whichsaid sEparation of said ash and recalcined lime particles is effectedphysically by mechanically removing said ash particles from said limeparticles based on and by utilizing the different sizes of suchparticles following discharge of the same from said furnace.
 9. Themethod of claim 1 in which said incineration and recalcination areeffected in a multiple-hearth furnace.
 10. The method of claim 9 inwhich the temperatures in said furnace are regulated to divide saidfurnace into three predetermined operative sludge treating zones inwhich the temperatures are maintained within preferred ranges asfollows: Entrance zone approximately 900*-1,100* F. Intermediate zoneapproximately 1,500*-1,900* F. Discharge zone approximately 800*-1,100*F.
 11. Apparatus for treating sewage sludge and lime sludge in a rawsewage treating procedure to incinerate and recalcine the samecomprising, in combination, A. a single furnace having input heatsources therein except adjacent the end thereof from which ash andrecalcined lime particles are discharged,
 12. The apparatus of claim 11which further includes C. a dewatering device for removing excess liquidfrom said sewage sludge and lime sludge prior to introduction thereofinto said furnace.
 13. The apparatus of claim 11 which further includesC. a slaker into which said ash and recalcined lime particles areintroducible for slaking and subsequent reuse of said lime particles insaid sewage treating procedure.
 14. The apparatus of claim 11 in whichsaid furnace is of the multiple hearth type having fuel burners alongeach of its hearths except the hearth adjacent said discharge end,whereby combustion and the carbon dioxide level adjacent said dischargeend are minimized to obviate recarbonation of the recalcined limedischarged from said furnace.
 15. The apparatus of claim 14 in whichsaid furnace is operatively divided into at least two distinct zones forincinerating and recalcining said sludge in which the temperatures aremaintained at a level not exceeding approximately 1,650* F. in a firstincinerating zone and maintained at approximately 1,850* F. in thesecond calcining zone.